1. Field of Invention
The invention relates to a tube expander and, in particular, to a tool that can expand a tube.
2. Related Art
The structure of a conventional tube expander is shown in FIG. 7. An axle 92 goes through the body 91 that is installed with a driving head 93. The lower part 911 of the body 91 has a guiding groove 912 for a pinch block 94 to enter. The body 91 has a through hole 913 going upward through the body 91 and downward to the guiding groove 912. At the top end of the through hole 913, the body 91 has an internal thread 914. The bottom end of the through hole 913 is provided with a bearing 95. The axle 92 inside the body 91 has a tapered head 921. The driving head 93 has an external thread 931 to engage the internal thread 914. As the driving head 93 rotates, the external thread 931 rotates with respect to the internal thread 914 so that the axle 92 displaces as it rotates. As the tapered head 921 deviates from the center, the tube 96 pinched by the pinch block 94 is expanded.
According to the above-mentioned structure of conventional tube expander, as the external thread 931 of the driving head 93 rotates into the internal thread 914 to the end, the continuing rotation of the tapered head 921 will continuously impose a pressure on the tube 96. In this case, the tube 96 will be deformed due to improper pressure. Therefore, a torque setting device is provided on the driving head 93 in the conventional tube expander. In this case, as the external thread 931 of the driving head 93 rotates into the internal thread 914 to the end, no further pressure is imposed on the tube 96 by the tapered head 921 even if the driving head 93 continues to rotate. This ensures that the tube 96 is not deformed by improper pressure.
Nonetheless, there is often another problem in practice. After the tube expansion is done, one has to reverse the driving head 93 for the external thread 931 to back out of the internal thread 914. Suppose the driving head 93 is driven by a hand tool with a larger torque, such as a motor-powered or air-powered screwdriver. If the driving head 93 is rotated out of the internal thread 914 to the limit without stopping in time, the body 91 will also rotate under the driving force of the driving head 93. If tthe user holds the body 91 by hand, his or her hand is likely to be twisted and hurt. If the body 91 is fixed by a vise, the hand tool may be over-worn by the twist and thus have a shorter lifetime.